CURLOPT_HEADERFUNCTION - callback that receives header data
#include <curl/curl.h>
size_t header_callback(char *buffer,
size_t size,
size_t nitems,
void *userdata);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADERFUNCTION,
header_callback);
Pass a pointer to your callback function, which should match the prototype shown
above.
This function gets called by libcurl as soon as it has received header data. The
header callback will be called once for each header and only complete header
lines are passed on to the callback. Parsing headers is easy to do using this
callback.
buffer points to the delivered data, and the size of that
data is
nitems;
size is always 1. Do not assume that the header
line is null-terminated!
The pointer named
userdata is the one you set with the
CURLOPT_HEADERDATA(3) option.
Your callback should return the number of bytes actually taken care of. If that
amount differs from the amount passed to your callback function, it will
signal an error condition to the library. This will cause the transfer to get
aborted and the libcurl function used will return
CURLE_WRITE_ERROR.
You can also abort the transfer by returning CURL_WRITEFUNC_ERROR. (7.87.0)
A complete HTTP header that is passed to this function can be up to
CURL_MAX_HTTP_HEADER (100K) bytes and includes the final line
terminator.
If this option is not set, or if it is set to NULL, but
CURLOPT_HEADERDATA(3) is set to anything but NULL, the function used to
accept response data will be used instead. That is, it will be the function
specified with
CURLOPT_WRITEFUNCTION(3), or if it is not specified or
NULL - the default, stream-writing function.
It's important to note that the callback will be invoked for the headers of all
responses received after initiating a request and not just the final response.
This includes all responses which occur during authentication negotiation. If
you need to operate on only the headers from the final response, you will need
to collect headers in the callback yourself and use HTTP status lines, for
example, to delimit response boundaries.
For an HTTP transfer, the status line and the blank line preceding the response
body are both included as headers and passed to this function.
When a server sends a chunked encoded transfer, it may contain a trailer. That
trailer is identical to an HTTP header and if such a trailer is received it is
passed to the application using this callback as well. There are several ways
to detect it being a trailer and not an ordinary header: 1) it comes after the
response-body. 2) it comes after the final header line (CR LF) 3) a Trailer:
header among the regular response-headers mention what header(s) to expect in
the trailer.
For non-HTTP protocols like FTP, POP3, IMAP and SMTP this function will get
called with the server responses to the commands that libcurl sends.
libcurl does not unfold HTTP "folded headers" (deprecated since RFC
7230). A folded header is a header that continues on a subsequent line and
starts with a whitespace. Such folds will be passed to the header callback as
a separate one, although strictly it is just a continuation of the previous
line.
Nothing.
Used for all protocols with headers or meta-data concept: HTTP, FTP, POP3, IMAP,
SMTP and more.
static size_t header_callback(char *buffer, size_t size,
size_t nitems, void *userdata)
{
/* received header is nitems * size long in 'buffer' NOT ZERO TERMINATED */
/* 'userdata' is set with CURLOPT_HEADERDATA */
return nitems * size;
}
CURL *curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);
curl_easy_perform(curl);
}
Always
Returns CURLE_OK
curl_easy_header(3),
CURLOPT_HEADERDATA(3),
CURLOPT_WRITEFUNCTION(3),